1. Field of the Invention
This invention relates to a bearing device having a double-row bearing, and particularly to a small bearing device that is used in the spindle motor, swing arm, IC cooling fan motor or the like for an information storage device such as a hard-disk drive, floppy-disk drive, or optical-disk drive.
2. Description of the Related Art
Conventionally, two ball bearings, that are spaced by a spacer, have been used as the bearing device for a hard-disk drive. Moreover, in Japanese Patent Publication No. Toku kai hei 10-227310, a bearing device with the object of reducing the outer diameter of the bearing while maintaining sufficient strength and shaft rigidity has been proposed. This publication is incorporated in this specification by reference.
In this kind of bearing device, the shaft is formed with two sections; a large diameter section and a small diameter section, and by forming an inner-ring raceway in the outer peripheral surface of the large diameter shaft section, the shaft is also used as one inner ring. In addition, the other inner ring is attached to the small-diameter shaft section.
Recently, as the motor of hard-disk drives has become smaller, it is necessary that the width of the bearing (dimension in the axial direction) be made smaller, for example, 2 mm or less. It has been thought that as a method for greatly reducing the width of the bearing, a single-row bearing could be used, however, when using a single-row bearing, vibration or run-out of the rotating shaft increases and it is not desirable.
A bearing unit that is capable of reducing the width of the bearing while at the same time suppressing vibration or run-out of the rotating shaft has been disclosed in Japanese Patent Publication No. Toku Kai Hei 6-222733, for example. This publication is incorporated in this specification by reference.
This bearing comprises two rows of balls, an inner ring, middle ring and outer ring, where the inner peripheral surface of the middle ring is the outer ring of one row of balls, and the outer peripheral surface of the middle ring becomes the inner ring of the other row of balls. In other words, the diameter of the pitch circle of the balls provided between the middle ring and the outer ring is larger than the diameter of the pitch circle of the balls provided between the inner ring and the middle ring. Moreover, the distance in the axial direction of the two rows of balls, specifically the distance between the centers of the balls is about the diameter of the balls.
Also, in the case where the double-row ball bearing is set between the shaft and the housing such that it is pre-loaded by a fixed-position pre-loading method, this pre-loading is performed by locating the two inner rings with the gap for adjusting the pre-load, and by pressing the sides of the two outer rings such that they are elastically deformed.
However, in the bearing of the aforementioned disclosure, the method of reducing the width of the bearing should be improved. In addition, in the bearing of the aforementioned disclosure, in order to apply a pre-load to the bearing, a complicated procedure is required when placing the bearing between the shaft and housing.
In a bearing with a small width (dimension in the axial direction) of 2 mm or less, for example, when two inner rings and outer rings with the same width are formed as in the prior double-row bearing, and the bearing is formed with a constant width in the radial direction, the bond margin is not sufficient, and there is the possibility that sufficient bond strength between the shaft and the housing cannot be obtained.
The double-row bearing is set between the shaft and the housing such that it is pre-loaded by a fixed-position pre-loading method, however, when it is not possible to obtain sufficiently strong bond strength, as described above, there is the possibility that the position of the two inner rings (races located with a gap for adjusting the pre-load) will not be fixed, and thus cause the space for adjusting the pre-load, as well as the pre-load, to change.
For example, the HDD used for a computer memory device comprises a housing fixed to a frame etc. and a support shaft fixed to the housing, and a bearing device for rotatably supporting a hub or inner member.
A single annular hard disc (or discs) has an inner peripheral portion supported by the hub or inner member to rotate together with the hub or inner member.
Since the hard disc has a thin track for recording data, the hub or inner member must be rotatably supported by the support shaft preventing vibration or NRRO (non-repetitive run out). Therefore, conventionally, as the bearing device provided e.g. between the outer peripheral surface of the support shaft and the inner peripheral surface of the hub, the structure where a pair of ball bearings are combined with each other, such that a preload is applied to the balls in the bearings so as to sufficiently secure the bearing rigidity.
JP Patent Publication Toku Kai Hei 10-159843 discloses such a bearing device. This publication is incorporated in this specification by reference.
In this structure, the support shaft has a base portion fixed to the center portion of the housing, and the bearing device has a pair of ball bearings are provided between the tip end and intermediate portion of the support shaft and the radially inner cylindrical portion within the hub. The inner races of the bearings are fixed through interference fit or adhesion to the tip end and intermediate portion of the shaft. The outer races are formed with cylindrical projections provided at an axial end and axially projecting than the end surfaces of the inner races. The tip ends of the projections are abutted to each other, and only the abutment portions of the outer races are fitted into the radially inner cylindrical portion of the hub. There are lands circumferentially provided on the radially inner side of the axially intermediate portion of the radially inner cylindrical portion.
In this bearing device, a desired preload is applied to the balls by pushing the inner races to come closer to each other with a gap provided between the axial end surfaces thereof. And, the hub is rotatably supported around the support shaft with no play under the preload.
Incidentally, when the bearing device under preload is incorporated in a narrow space, generally the bearing device is preloaded through the fixed position preloading method. There are two ways for mounting the bearing device preloaded through the fixed position preloading method to the support shaft.
1. a pair of inner races are fitted onto and fixed to the support shaft through interference fit applying a load to have the inner races come closer to each other.
2. a pair of inner races are fitted onto the support shaft through clearance fit, and bonded for fixing under a load to have the inner races to come closer to each other.
The installation of the bearing device to the shaft in either way above can be carried out only by the bearing manufacturer who has a special apparatus to highly precisely apply a desired preload to the bearing device.
Taking the problems with the prior art into consideration, the object of this invention is to provide a double-row bearing that is set between the shaft and the housing such that it is pre-loaded by a fixed-position pre-loading method, and that will make it possible to greatly reduce the width of the bearing to 2 mm or less while at the same time suppress vibration of the rotating shaft, as well as reduce the number of components and make it possible to perform pre-loading more easily when setting the bearing between the shaft and the housing.
Moreover, another object of the invention is to provide a bearing device having a shaft, a housing and a double-row bearing that is set between the shaft and housing such that it is pre-loaded by a fixed-position pre-loading method, and that prevents movement of the two races, that are located with the gap for adjusting the pre-load, such that no change occurs in the gap for adjusting the pre-load, even when the width (dimension in the axial direction) of the double-row bearing is extremely small or less than 2 mm.
Another object of the present invention is to provide a small lightweight bearing device having sufficient rigidity under preload, through reduced manufacturing steps for cost reduction, and for a long durability of performance.